<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>table练习</title>
  <style>
    .active{
      color: red;
      font-size: 24px;
      background-color: green;
    }
  </style>
</head>
<body>
  <div class="box">
    我是box
  </div>

  <script>
    var boxEl = document.querySelector(".box");
    boxEl.onclick = function(){
      boxEl.className = "active";
    }
  </script>
  
</body>
</html>